In a process for producing acrolein, a gas produced by a reaction of forming acrolein (hereinafter referred to as "reaction gas") containing acrolein, acrylic acid, water and others is generally sufficiently cooled by direct contact with a condensate in a packed tower or the like thereby to condense the acrylic acid, water and others, subsequently the acrolein remaining in the resulting gas phase is separated from the nitrogen and other gases by being absorbed in water or the like, and the absorbed acrolein is then stripped. Thus, high-purity acrolein is recovered.
The condensate contains acrolein and acrylic acid each in an amount of several percents by weight and further contains about 1% by weight of acrolein hydrate (hydroxypropionaldehyde). Since discarding the condensate necessitates costly wastewater treatment and results in deteriorated unit requirement of propylene, the acrolein and acrylic acid are usually recovered from the condensate. In order to recover the acrolein, the condensate is heated to strip the acrolein, which is then recovered along with the gas-phase acrolein described above. The remaining acrylic acid is recovered by extracting with a solvent and then separating from the solvent by distillation.
If the condensate, from which acrylic acid is to be recovered, contains an acrolein hydrate (hydroxypropionaldehyde) in a large amount, acrolein comes into the recovered acrylic acid to lower the purity of the acrylic acid.
Although the acrolein hydrate can be decomposed by elevating the temperature, this method is not preferred in that such high temperatures result in sludge formation and necessitate removal thereof.